PROJECT SUMMARY Adolescence is the period during which youth begin to explore ?adult behaviors,? including cannabis use. Most of these exploratory behaviors occur in the context of peers. These emergent behaviors coincide with profound neurodevelopmental changes that occur with the onset of puberty, wherein the brain has an enhanced period of exploration. Increased social focus is one byproduct, whereby adolescents spend more time with peers and show heightened emotional investment in peer evaluations and peer experiences. Indeed, most youth begin exploring with cannabis in peer contexts and are highly influenced by peer use, which are likely exacerbated in states with legal recreational cannabis use, including Oregon and Washington. The goal of the proposed study is to use an innovative social neuroscience approach to elucidate the intersection of peer behaviors and neurobiological factors involved in adolescent cannabis use. The project will employ magnetic resonance imaging (MRI) to examine how brain response differs by peer social context. To achieve this aim, 80 adolescents (14-19 year-olds, n=40 heavy cannabis users, n=40 cannabis-nave youth) will complete MRI resting state and fMRI social influence tasks in two different peer context conditions (alone vs. with a co- participating friend). This research will provide valuable foundational data for future investigations of how best to address the impact of peer use in cannabis prevention and intervention programming.